Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

At least one exemplary embodiment is directed to an optical scanning
apparatus which includes a Vertical Cavity Surface Emitting Laser
including a plurality of light-emitting portions that are spaced from
each other in at least a sub-scanning direction, a first optical system
including a light-condensing element that converts each of light beams
from the laser into a light beam in another state; a deflector that
reflects and deflects the light beams from the first optical system, and
a second optical system that focuses the light beams deflected by the
deflecting member on a surface to be scanned, where the second optical
system includes at least an imaging optical element having an optical
surface with a non-arc shape in a sub-scanning cross section.

Claims:

1. An optical scanning apparatus comprising:a Vertical Cavity Surface
Emitting Laser including a plurality of light-emitting portions that are
spaced from each other in at least a sub-scanning direction, wherein the
plurality of light-emitting portions emits a plurality of light beams;a
first optical system including a light-condensing element that converts
the plurality of light beams into a combined light beam in another
state;a deflecting unit that reflects the combined light beam; anda
second optical system that focuses the reflected combined light beam on a
surface to be scanned, the second optical system including an imaging
optical element having an optical surface with a non-arc shape in a
sub-scanning cross section,wherein a principal ray of a light beam
emitted from one of the plurality of light-emitting portions that is
farthest from an optical axis in the sub-scanning cross section passes
through a plurality of optical elements included in the first and second
optical systems, the principal ray being farthest from the optical axis
in the sub-scanning cross section when the principal ray passes through
the optical surface of the imaging optical element, andwherein, when the
focal length in the sub-scanning direction of the light-condensing
element is Fcol (mm), the distance between the optical axis and the
light-emitting portion that is farthest from the optical axis in the
sub-scanning cross section is Lo, the distance between the optical
surface of the imaging optical element and the deflecting unit along the
optical axis direction is SI, the imaging magnification of the first
optical system in the sub-scanning direction is β0, and the
F-number of the entrance side of the light-condensing element in the
sub-scanning cross section is Fno, the following expression is
satisfied:0.10<|(SI/Fcol+β0)×L0/(SI/(Fno×.b-
eta..sub.0.times.2)|<5.43.

2. An image-forming apparatus comprising:an optical scanning apparatus
according to claim 1, which emits light beams;a photosensitive body
disposed on the surface to be scanned;a developing device that forms a
toner image by developing an electrostatic latent image formed on the
photosensitive body by the light beams emitted from the optical scanning
apparatus;a transferring device that transfers the toner image onto a
transferring material; anda fixing device that fixes the toner image
transferred onto the transferring material.

3. An image-forming apparatus comprising:an optical scanning apparatus
according to claim 1; anda printer controller that converts code data
received from an external device into an image signal and inputs the
image signal to the optical scanning apparatus.

4. A color-image-forming apparatus comprising:a plurality of the optical
scanning apparatus according to claim 1; anda plurality of image carriers
respectively arranged on the surface to be scanned of the optical
scanning apparatus and forming images of different colors.

5. The color-image-forming apparatus according to claim 4, further
comprising a printer controller that converts color signals input from an
external device into color image data elements and inputs the color image
data elements to the respective optical scanning apparatus.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001]This application is a continuation application pursuant to 37 CFR
§1.53(b) of U.S. application Ser. No. 12/196,986 filed Aug. 22,
2008, which is a divisional of U.S. application Ser. No. 11/400,673 filed
Apr. 7, 2006 and issued as U.S. Pat. No. 7,439,999, which claims the
benefit of Japanese Application No. 2005-132579 filed Apr. 28, 2005, all
of which are hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to an optical scanning apparatus and
an image-forming apparatus using the optical scanning apparatus.

[0006]In this type of optical scanning apparatus, a light beam emitted
from a light source unit including a semiconductor laser is collimated by
a collimating lens and is guided to a deflecting-reflecting surface
(deflecting surface) of a light deflector including a rotating polygon
mirror. The light beam deflected by the light deflector is caused to form
a spot image on a surface by an imaging optical system (fθ lens
system), and this surface is scanned with the light beam at a constant
speed. This type of optical scanning apparatus includes a so-called
surface-tilt-correction optical system in which the substantially
collimated light beam output from the collimating lens is collected at or
near the deflecting-reflecting surface in a sub-scanning direction
(sub-scanning cross section) perpendicular to the deflecting direction
(main-scanning direction) by a cylindrical lens and is then caused to
form the spot image on a surface to be scanned by the imaging optical
system.

[0007]Recently, demand for high-speed and high-definition printing
performance of image-forming apparatuses (e.g., laser beam printers,
digital copy machines, and multifunction printers) has increased. To
achieve either high-speed printing or high-definition printing, it can be
necessary in some circumstances to increase the number of times the
surface is scanned per unit time. Accordingly, the number of surfaces and
the rotating speed of the rotating polygon mirror have been increased.

[0008]However, in this case, the size of the rotating polygon mirror and
load placed on a drive motor are increased. Therefore, new problems arise
that the temperature and noise are increased and the overall size cannot
be reduced.

[0009]Accordingly, in order to reduce the load placed on the light
deflector, various types of multi-beam scanning methods have been
suggested in which the number of light-emitting portions included in a
semiconductor laser that can function as the light source unit is
increased and a plurality of light beams are contemporaneously deflected
and caused to scan a surface to be scanned.

[0010]There are two major types of light sources used in the multi-beam
scanning methods:

[0011]A first type in which a plurality of light-source elements which
each emit a single laser beam can be arranged and a plurality of light
beams can be obtained using optical-path-combining units (e.g.,
polarizing beam splitters and half mirrors); and

[0012]A second type called a monolithic multi-beam type in which a
plurality of light-emitting portions can be arranged on a single
light-source element.

[0013]Although light sources of the first type can be easily manufactured
using simple (inexpensive) single laser emitting elements, there is a
problem that the overall structure is complex and large because the
beam-combining units are necessary. In comparison, in the monolithic
multi-beam type, no beam-combining unit is necessary and accordingly the
structure of the optical scanning apparatus can be made simpler and
smaller.

[0014]There are two major types of monolithic multi-beam light-source
elements: horizontal emission type and vertical emission type. Each type
of light-source element is manufactured by a semiconductor process and
has a layered structure formed on a wafer substrate. The beam is emitted
horizontally from the layered structure in the horizontal emission type
and vertically in the vertical emission type.

[0015]In general, semiconductor lasers of the horizontal emission type are
mainly used because they can be easily manufactured. In multi-beam light
sources of the horizontal emission type, beams can be arranged
one-dimensionally. The horizontal emission type is also called an edge
emitter type.

[0016]In the vertical emission type, light-emitting portions can be
arranged two-dimensionally on the substrate surface because the light
beams are emitted vertically with respect to the substrate surface.
Accordingly, the laser sources of this type are called Vertical Cavity
Surface Emitting Lasers. The Vertical Cavity Surface Emitting Lasers are
advantageous in that the number of light-emitting portions can be easily
increased by arranging them two-dimensionally, and have recently been
attracting considerable attention.

[0017]On the other hand, optical elements included in imaging lenses of
the optical scanning apparatus are generally formed by molding using a
mold. Molding is advantageous in that lenses having complex shapes can be
easily manufactured with high reliability once the mold is obtained.
Accordingly, optical elements having aspherical surfaces are often
manufactured by molding so that the optical performance can be increased
and the number of lenses can be reduced. In particular, various lens
structures having surfaces aspherical in the main-scanning direction have
been suggested to reduce the comma aberration and improve the fθ
characteristics.

[0018]In addition, various kinds of optical scanning apparatuses including
lens surfaces aspherical in the sub-scanning direction have also been
suggested (see Japanese Patent Laid-Open Nos. 2001-021824, 2-157809,
9-90254, 2000-121977, and 2004-70108).

[0022]The structures according to Japanese Patent Laid-Open Nos.
2001-021824, 2-157809, and 9-90254 compensate for a displacement between
a paraxial image plane and a best-spot image plane caused by the
influence of the spherical aberration generated due to an increase in the
width of the light beam in the sub-scanning direction.

[0023]In the structures according to Japanese Patent Laid-Open Nos.
2000-121977 and 2004-70108, the light beam incident at an angle passes
through an imaging lens surface at a position separated from the optical
axis in the sub-scanning cross section. Accordingly, the irradiation
height of the image point on the surface to be scanned is largely shifted
from the optical axis due to the spherical aberration of the imaging
lens, which generates the scan-line curvature. The above-mentioned
structures are provided to reduce this scan-line curvature.

[0024]The above-described Vertical Cavity Surface Emitting Laser that
emits an increased number of beams from two-dimensionally arranged
light-emitting portions can have a certain field angle to reduce jitter
in the main scanning direction.

[0025]The jitter in the main-scanning direction will be explained below.
Since the light-emitting portions included in the laser chip are
separated from each other and gaps with a certain width are provided
between the spots in the main-scanning direction, two light beams
propagate at an angle with respect to each other in the main-scanning
direction. Accordingly, the polygon mirror is at different rotational
positions when the two light beams are incident on (scan) the same point
on the photosensitive drum in the main-scanning direction, which means
that the two light beams can be incident on that point at different
times. Therefore, the positions (distances from the optical axis) at
which the two light beams pass through the imaging lens (fθ lens)
also differ from each other in the main-scanning direction, and
sufficient effects cannot be obtained due to differences between the
positions at which the light beams pass through the imaging lens in the
main-scanning direction. In other words, since the principal rays of the
light beams are incident on the polygon mirror at different positions,
the light beams that travel toward the same image height in the
main-scanning direction pass through the imaging lens at different
positions. This causes the jitter in the main-scanning direction.

[0026]The jitter in the main-scanning direction can be reduced by
arranging the laser source such that the field angle in the main-scanning
direction is reduced, that is, such that the field angle in the
sub-scanning direction is increased.

[0027]However, when the field angle in the sub-scanning direction is
increased, the following problems occur:

[0028]A field curvature between the beams occurs in the sub-scanning cross
section; and

[0029]The gaps between the beams become uneven due to a distortion (DIST)
in the sub-scanning cross section.

[0030]For example, FIGS. 21 and 22 show aberrations obtained when a
collimating lens with a focal length (F) of 16.3 and a cylindrical lens
with a focal length (F) of 36.0 in the sub-scanning direction are
included in the incident optical system according to Japanese Patent
Laid-Open No. 2003-156704 and a Vertical Cavity Surface Emitting Laser
having a field angle in the sub-scanning direction is used as a laser
source.

[0031]FIG. 21 illustrates a graph of the paraxial image plane in the
sub-scanning direction, where the vertical axis illustrates the paraxial
image plane in the sub-scanning direction (sub-scan image plane) and the
horizontal axis illustrates the image height (scan image height) on a
surface to be scanned in the main-scanning direction. The graph
illustrates the case in which the light-emitting portions of the laser
source can be arranged such that the field angle is varied with 0.02 mm
pitch in the range of Z=0.000 mm to 0.100 mm in terms of the distance
from the optical axis of the collimating lens in the sub-scanning
direction.

[0032]As is clear from FIG. 21, as the field angle in the sub-scanning
direction (sub-scan field angle) of the light-emitting portions is
increased, the sub-scan image plane is shifted in the negative direction
and accordingly a field curvature occurs.

[0033]The sub-scan image plane is particularly largely shifted in a region
where the scan image height is near the axis. The sub-scan image plane is
curved with respect to the sub-scan field angle (Z=0.000 mm to 0.100 mm
for the field angle of the laser source).

[0034]Although the shift of the sub-scan image plane is small if the
sub-scan field angle is small (around Z=0.02 mm for the field angle of
the laser source), it cannot be ignored when the Vertical Cavity Surface
Emitting Laser is used and the sub-scan field angle is increased.

[0035]FIG. 22 illustrates a graph of the irradiation height of the image
point on a surface to be scanned in the sub-scanning direction, where the
horizontal axis illustrates the image height on the surface to be scanned
in the main-scanning direction (scan image height) and the vertical axis
illustrates the irradiation height of the image point in the sub-scanning
direction. The graph illustrates the case in which the light-emitting
portions of the laser source can be arranged such that the field angle is
varied with 0.02 mm pitch in the range of Z=0.000 mm to 0.100 mm in terms
of the distance from the optical axis of the collimating lens in the
sub-scanning direction.

[0036]As is clear from FIG. 22, when the sub-scan field angle of the
light-emitting portions is large, the irradiation height of the image
point in the sub-scanning direction is shifted in the positive direction
as the image height in the main scanning direction is increased.
Accordingly, a scan-line curvature occurs.

[0037]This means that the gap between the beams in the sub-scanning
direction (sub-scan pitch) varies depending on the main-scan image
height.

[0038]The amount of variation is particularly large in regions where the
scan image height is large, and distortion (DIST) in the sub-scanning
direction occurs in these regions.

[0039]Although the variation in the sub-scan pitch between the beams is
small if the sub-scan field angle is small (around Z=0.02 mm for the
field angle of the laser source), it cannot be ignored when the Vertical
Cavity Surface Emitting Laser is used and the sub-scan field angle is
increased.

[0040]Japanese Patent Laid-Open No. 2001-021824 discusses an aberration
correction structure for a light source emitting a plurality of beams
arranged such that the light source has a field angle in the sub-scanning
direction. However, in this structure, the field angle in the
sub-scanning direction is assumed to be around ±0.021 mm or less,
which corresponds to the cases where the field angle is very small in the
graphs shown in FIGS. 21 and 22.

[0041]In addition, the specification of Japanese Patent Laid-Open No.
2001-021824 discusses no concept for compensating for the differences in
aberrations between the beams.

[0042]In addition, there is another problem in that the influence of wave
aberration is increased when the beam diameter in the sub-scanning
direction is increased to reduce the spot size.

[0043]To correct this, in the structure according to Japanese Patent
Laid-Open No. 2001-021824, a surface where the beam diameter in the
sub-scanning direction is at a maximum is designed to be aspherical.
However, this is not sufficient for a light source arranged to have a
field angle in the sub-scanning direction.

[0044]This is because a light beam with a field angle in the sub-scanning
direction can cause a coma aberration when the light beam passes through
an optical surface at a position separated from the optical axis and the
coma aberration is increased as the light beam width is increased.

[0045]Therefore, the comma aberrations caused by light beams that pass
through a lens surface at different positions cannot be sufficiently
reduced by the structure according to Japanese Patent Laid-Open No.
2001-021824.

SUMMARY OF THE INVENTION

[0046]At least one exemplary embodiment is directed to an optical scanning
apparatus that can be used in an image-forming apparatus (e.g., a laser
beam printer (LBP), a digital copy machine, and a multifunction printer
that performs electrophotography processes, and other image-forming
apparatus as conventional by one of ordinary skill in the relevant arts
and equivalents).

[0047]At least one exemplary embodiment is directed to an optical scanning
apparatus that reliably corrects and/or reduces aberrations including
field curvature and distortion (DIST) in a sub-scanning cross section to
provide good/improved optical performance and an image-forming apparatus
using the optical scanning apparatus.

[0048]At least one exemplary embodiment is also directed to an optical
scanning apparatus using a Vertical Cavity Surface Emitting Laser as a
light source unit and forming an image by contemporaneously deflecting
and scanning a plurality of light beams, which can have a large field
angle in the sub-scanning direction, and an image-forming apparatus using
the optical scanning apparatus.

[0049]According to an exemplary embodiment of the present invention, an
optical scanning apparatus includes a Vertical Cavity Surface Emitting
Laser including a plurality of light-emitting portions that are spaced
from each other in at least a sub-scanning direction; a first optical
system including a light-condensing element that converts each of light
beams from the laser source into a light beam in another state; a
deflector that reflects and deflects the light beams from the first
optical system; and a second optical system that focuses the light beams
deflected by the deflecting member on a surface to be scanned, the second
optical system including at least an imaging optical element having an
optical surface with a non-arc shape in a sub-scanning cross section.
When the number of the light-emitting portions is N, the focal length of
the light-condensing element is Fcol (mm), the maximum effective image
circle diameter of the light-condensing element is IS (mm), the imaging
magnification of the second optical system in the sub-scanning direction
is βFθ, and the distance between the light beams on the a
surface to be scanned in the sub-scanning direction is 25.4/DPI (mm), the
following expression is satisfied:

0.18 (mm)≦(N-1)×Fcol/(IS×βFθ×DPI)-
≦12.00 (mm).

[0050]In the optical scanning apparatus of at least one exemplary
embodiment, the following expression can also be satisfied:

0.24 (mm)≦(N-1)×Fcol/(IS×βFθ×DPI)-
≦8.78 (mm).

[0051]The optical scanning apparatus can further include a diaphragm
disposed between the laser source and the deflector and an optical
surface of an optical element disposed between the diaphragm and the
deflector and being adjacent to the diaphragm can have a non-arc shape in
the sub-scanning cross section.

[0052]In the optical scanning apparatus, variation directions of field
curvatures caused by the first optical system and the second optical
system in the sub-scanning direction due to variation in a field angle in
the sub-scanning direction can be opposite to each other.

[0053]In addition, variation directions of distortions caused by the first
optical system and the second optical system in the sub-scanning
direction due to variation in a field angle in the sub-scanning direction
can be opposite to each other.

[0054]In addition, according to another exemplary embodiment of the
present invention, an optical scanning apparatus includes a Vertical
Cavity Surface Emitting Laser including a plurality of light-emitting
portions that are spaced from each other in at least a sub-scanning
direction; a first optical system including a light-condensing element
that converts each of light beams from the laser source into a light beam
in another state; a deflector that reflects and deflects the light beams
from the first optical system; and a second optical system that focuses
the light beams deflected by the deflecting member on a surface to be
scanned, the second optical system including an imaging optical element
having an optical surface with a non-arc shape in a sub-scanning cross
section. A principal ray of a light beam emitted from one of the
light-emitting portions, that is farthest from an optical axis in the
sub-scanning cross section, passes through a plurality of optical
elements included in the first and second optical systems, the principal
ray being farthest from the optical axis in the sub-scanning cross
section when the principal ray passes through the optical surface of the
imaging optical element. In addition, when the focal length of the
light-condensing element is Fcol (mm), the distance between the optical
axis and the light-emitting portion that is farthest from the optical
axis in the sub-scanning cross section is L0, the distance between
the optical surface of the imaging optical element and the deflector
along the optical axis direction is SI, the imaging magnification of the
first optical system in the sub-scanning direction is β0, and
the F-number of the entrance side of the light-condensing element in the
sub-scanning cross section is Fno, the following expression can be
satisfied:

0.10<|(SI/Fcol+β0)×L0/(SI/(Fno×β0.-
times.2)|<5.43.

[0055]According to another exemplary embodiment of the present invention,
an optical scanning apparatus includes a Vertical Cavity Surface Emitting
Laser including a plurality of light-emitting portions that are spaced
from each other in at least a sub-scanning direction; a first optical
system including a light-condensing element that converts each of light
beams from the laser source into a light beam in another state; a
deflector that reflects and deflects the light beams from the first
optical system; a diaphragm disposed between the laser source and the
deflector and having an optical surface with a non-arc shape in a
sub-scanning cross section; and a second optical system that focuses the
light beams deflected by the deflecting member on a surface to be
scanned. A principal ray of a light beam emitted from one of the
light-emitting portions that is farthest from an optical axis in the
sub-scanning cross section passes through a plurality of optical elements
included in the first and second optical systems, the principal ray being
farthest from the optical axis in the sub-scanning cross section when the
principal ray passes through the optical surface of the diaphragm.

[0056]Also in this optical scanning apparatus, variation directions of
field curvatures caused by the first optical system and the second
optical system in the sub-scanning direction due to variation in a field
angle in the sub-scanning direction can be opposite to each other.

[0057]In addition, variation directions of distortions caused by the first
optical system and the second optical system in the sub-scanning
direction due to variation in a field angle in the sub-scanning direction
can be opposite to each other.

[0058]In addition, according to another exemplary embodiment of the
present invention, an image-forming apparatus includes the
above-described optical scanning apparatus, a photosensitive body
disposed on the surface to be scanned; a developing device that can form
a toner image by developing an electrostatic latent image formed on the
photosensitive body by the light beams emitted from the optical scanning
apparatus; a transferring device that transfers the developed toner image
onto a transferring material; and a fixing device that fixes the toner
image transferred onto the transferring material.

[0059]In addition, according to another exemplary embodiment of the
present invention, an image-forming apparatus includes the
above-described optical scanning apparatus and a printer controller that
converts code data received from an external device into an image signal
and inputs the image signal to the optical scanning apparatus.

[0060]In addition, according to another exemplary embodiment of the
present invention, a color-image-forming apparatus includes a plurality
of the above-described optical scanning apparatus and a plurality of
image carriers respectively arranged on the surface to be scanned of the
optical scanning apparatus and forming images of different colors.

[0061]The color-image-forming apparatus can further include a printer
controller that converts color signals input from an external device into
color image data elements and inputs the color image data elements to the
respective optical scanning apparatus.

[0062]Accordingly, at least one embodiment of the present invention
provides is directed to an optical scanning apparatus that reduces
aberrations including field curvature and distortion (DIST) in the
sub-scanning cross section and provides good optical performance even
when a Vertical Cavity Surface Emitting Laser is used as a light source
unit and an image-forming apparatus including the optical scanning
apparatus.

[0063]Further features will become apparent from the following description
of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0064]FIG. 1A illustrates a main-scanning cross section according to a
first exemplary embodiment of the present invention.

[0065]FIG. 1B illustrates a sub-scanning cross section according to the
first exemplary embodiment of the present invention.

[0066]FIG. 2 illustrates a graph of the sub-scan image plane according to
the first exemplary embodiment of the present invention.

[0067]FIG. 3 illustrates a graph of the image-point irradiation height on
an image plane in a sub-scanning direction according to the first
exemplary embodiment of the present invention.

[0068]FIG. 4 illustrates a graph of the beam height in each surface
according to the first exemplary embodiment of the present invention.

[0069]FIG. 5 illustrates a main-scanning cross section according to a
second exemplary embodiment of the present invention.

[0070]FIG. 6 illustrates a graph of the sub-scan image plane according to
the second exemplary embodiment of the present invention.

[0071]FIG. 7 illustrates a graph of the image-point irradiation height on
an image plane in a sub-scanning direction according to the second
exemplary embodiment of the present invention.

[0072]FIG. 8 illustrates a graph of the beam height in each surface
according to the second exemplary embodiment of the present invention.

[0073]FIG. 9 illustrates a graph of the sub-scan image plane according to
a third exemplary embodiment of the present invention.

[0074]FIG. 10 illustrates a graph of the image-point irradiation height on
an image plane in a sub-scanning direction according to the third
exemplary embodiment of the present invention.

[0075]FIG. 11 illustrates a graph of the beam height in each surface
according to the third exemplary embodiment of the present invention.

[0076]FIG. 12 illustrates a main-scanning cross section according to a
fourth exemplary embodiment of the present invention.

[0077]FIG. 13 illustrates a graph of the sub-scan image plane according to
the fourth exemplary embodiment of the present invention.

[0078]FIG. 14 illustrates a graph of the image-point irradiation height on
an image plane in a sub-scanning direction according to the fourth
exemplary embodiment of the present invention.

[0079]FIG. 15 illustrates a graph of the beam height in each surface
according to the fourth exemplary embodiment of the present invention.

[0080]FIG. 16 illustrates a graph of the sub-scan image plane according to
a fifth exemplary embodiment of the present invention.

[0081]FIG. 17 illustrates a graph of the image-point irradiation height on
an image plane in a sub-scanning direction according to the fifth
exemplary embodiment of the present invention.

[0082]FIG. 18 illustrates a graph of the beam height in each surface
according to the fifth exemplary embodiment of the present invention.

[0083]FIG. 19 is a schematic diagram illustrating an image-forming
apparatus according to an exemplary embodiment of the present invention.

[0084]FIG. 20 is a schematic diagram illustrating a color image-forming
apparatus according to another exemplary embodiment of the present
invention.

[0085]FIG. 21 illustrates a graph of the sub-scan image plane according to
a conventional structure.

[0086]FIG. 22 illustrates a graph of the image-point irradiation height on
an image plane in a sub-scanning direction according to the conventional
structure.

DESCRIPTION OF THE EMBODIMENTS

[0087]The following description of at least one exemplary embodiment is
merely illustrative in nature and is in no way intended to limit the
invention, its application, or uses.

[0088]Processes, techniques, apparatus, and materials as conventional by
one of ordinary skill in the relevant art may not be discussed in detail
but are intended to be part of the enabling description where
appropriate, for example the fabrication of the lens and mirror elements
and their materials.

[0089]In all of the examples illustrated and discussed herein, any
specific values, for example pitch values and focal lengths, should be
interpreted to be illustrative only and no limiting. Thus, other examples
of the exemplary embodiments could have different values.

[0090]Notice that similar reference numerals and letters refer to similar
items in the following figures, and thus once an item is defined in one
figure, it may not be discussed for following figures.

[0091]Exemplary embodiments will be described below with reference to the
accompanying drawings.

First Exemplary Embodiment

[0092]FIG. 1A illustrates a cross section of the main part of an optical
scanning apparatus according to a first exemplary embodiment of the
present invention taken along a main-scanning direction (main-scanning
cross section) and FIG. 1B illustrates a cross section of the main part
of the optical scanning apparatus according to the first exemplary
embodiment of the present invention taken along a sub-scanning direction
(sub-scanning cross section).

[0093]The main-scanning direction refers to a direction perpendicular to a
rotating axis of, for example, a rotating polygon mirror and/or an
optical axis of an imaging optical system (that is, a direction in which
light beams are reflectively deflected (deflected and scanned) by the
rotating polygon mirror. The sub-scanning direction refers to a direction
parallel to the rotating axis of the rotating polygon mirror. The
main-scanning cross section refers to a plane including the main-scanning
direction and the optical axis of the imaging optical system and the
sub-scanning cross section refers to a plane perpendicular to the
main-scanning cross section.

[0094]The structure shown in FIGS. 1A and 1B and the optical operation
thereof will be described below.

[0095]Referring to the figures, a Vertical Cavity Surface Emitting Laser 1
includes a plurality of light-emitting portions that can be arranged in
the sub-scanning direction with gaps provided therebetween.

[0098]A lens system (cylindrical lens) 4 has a predetermined power in the
sub-scanning direction and causes the light beams that pass through the
aperture diaphragm 3 to form substantially linear images on a deflecting
surface (reflecting surface) 5a of a light deflector 5, which will be
described below, in the sub-scanning cross section.

[0099]The elements including the collimating lens 2, the aperture
diaphragm 3, and the cylindrical lens 4 are included in a first optical
unit (incident optical system) LA. The functions of the collimating lens
2 and the cylindrical lens 4 can also be obtained by a single optical
element (anamorphic lens).

[0100]The light deflector 5 can function as a deflecting unit and
includes, for example, a four-surface rotating polygon mirror inscribed
in a φ20 circle (circle with a diameter of 20 mm). The light
deflector 5 is rotated by a driving unit (not shown), such as a motor, at
a constant speed in the direction shown by the arrow A. In the present
non-limiting example of at least one exemplary embodiment, the width of
the deflecting-reflecting surfaces (deflecting surfaces) 5a of the
polygon mirror 5 in the main-scanning direction is 14.1 mm.

[0101]An imaging optical system (fθ lens system) 6 can function as a
second optical unit and includes first and second imaging lenses 61 and
62 (e.g., made of resin (plastic)). The imaging optical system 6 causes
the light beams based on image information that are reflected and
deflected by the light deflector 5 to form images on a photosensitive
drum surface 7 that can function as a surface to be scanned. In addition,
the imaging optical system 6 performs surface-tilt correction by
establishing a conjugate relationship between the deflecting surface 5a
of the light deflector 5 and the photosensitive drum surface 7 in the
sub-scanning cross section.

[0102]The first and second imaging lenses 61 and 62 (e.g., made of resin)
can be manufactured by several conventional processes (e.g., a molding
process in which resin is injected into a mold and is taken out from the
mold after being cooled). Accordingly, if the imaging lenses 61 and 62
are made of moldable material they can be easily manufactured with low
cost compared to conventional imaging lenses made of glass.

[0103]The first imaging lens 61 can have a positive power mainly in the
main-scanning direction, as illustrated in Table 1-1 which will be
described below, and has aspherical lens surfaces with shapes expressed
by Equations (a) to (d) which will also be described below.

[0104]The first imaging lens 61 has a higher power in the main-scanning
cross section (main-scanning direction) than in the sub-scanning cross
section (sub-scanning direction). In the main-scanning cross section, the
first imaging lens 61 can have a meniscus shape and the entrance surface
thereof is non-arc and concave toward the light deflector 5. In the
sub-scanning cross section, the first imaging lens 61 can have a
cylindrical shape in which both the entrance and exit surfaces are flat
in the sub-scanning direction. However, it is not necessary that the
entrance and exit surfaces be completely flat, and the first imaging lens
61 can also have a certain power in the sub-scanning cross section.

[0106]The second imaging lens 62 is an anamorphic lens having different
powers in the main-scanning direction and the sub-scanning direction, as
illustrated in Table 1-1 which will be described below. The entrance and
exit surfaces of the second imaging lens 62 are aspherical surfaces
having shapes corresponding to Expressions A and B, respectively, as
illustrated in Table 1-1. The exit surface has a non-arc shape in the
sub-scanning cross section.

[0107]The second imaging lens 62 can have a higher power in the
sub-scanning cross section than in the main-scanning cross section. In
the main-scanning cross section, the entrance surface of the second
imaging lens 62 has an arc shape and the exit surface thereof has a
non-arc shape.

[0108]The second imaging lens 62 has a lens shape that is asymmetric with
respect to the optical axis in the main-scanning cross section, and has
substantially no power in the main-scanning direction in a region around
the optical axis. In the sub-scanning cross section, the entrance surface
of the second imaging lens 62 has a concave shape with a small curvature.
In addition, the exit surface has a non-arc convex shape with a curvature
that gradually changes as the distance from the optical axis is
increased, and is asymmetric with respect to the optical axis.

[0109]The second imaging lens 62 focuses the light beams incident thereon
mainly in the sub-scanning direction. In addition, the second imaging
lens 62 also serves a certain distortion-correcting function in the
main-scanning direction.

[0110]It is not necessary to express the shapes of the first and second
imaging lenses 61 and 62 with the functional expressions using the
aspherical values shown in Table 1-1, and other conventional expressions
or equivalent expressing methods can also be used. In addition, it is not
necessary that the first and second imaging lenses 61 and 62 be symmetric
or asymmetric with respect to the optical axis as illustrated in Table
1-1, and other conventional structures can also be applied.

[0111]The photosensitive drum surface 7 can function as the surface to be
scanned.

[0112]Table 1-1 shows data of the optical scanning apparatus according to
the present exemplary embodiment. The unit of length is mm, the unit of
angle is degree, and the unit of resolution is dot/inch. This applies to
a substantial portion of the following exemplary embodiments.

Surface Shapes of First and Second Imaging Lenses 61 and 62: Expression A

[0113]Expression A that defines the surface shapes of the first and second
imaging lenses 61 and 62 is determined as described below.

[0114]When a surface has an aspherical shape that can be expressed by a
function of tenth or lower order and when an intersection of the surface
and the optical axis is the origin, x axis extends in the optical-axis
direction, y axis extends perpendicular to the optical axis in the
main-scanning plane, and the z axis extends perpendicular to the x axis
in the sub-scanning plane, the shape of the surface in a meridional
direction, which corresponds to the main-scanning direction, is expressed
as follows:

where R is the radius of curvature and K, B4, B6, B8, and
B10 are aspherical surface coefficients.

[0115]In addition, the shape of the surface in a sagittal direction, which
corresponds to the sub-scanning direction (direction perpendicular to the
optical axis and the main-scanning direction), is expressed as follows:

S = Z 2 / r ' 1 + ( 1 - ( Z / r ' ) 2 ) 1 / 2
( b ) ##EQU00002##

where r' is calculated as
r'=rC(1+D2Y2+D4Y4+D6Y6+D8Y8+-
D10Y10, and where r0 is the radius of curvature in the
sagittal direction on the optical axis and D2, D4, D6,
D8, and D10 are coefficients.

Surface Shape of Second Imaging Lens 62: Expression B

[0116]Expression B that defines the surface shape of the second imaging
lens 62 which can have an aspherical surface in the sub-scanning cross
section is determined as described below.

[0117]When a surface has an aspherical shape that can be expressed by a
function of tenth or lower order and when an intersection of the surface
and the optical axis is the origin, x axis extends in the optical-axis
direction, y axis extends perpendicular to the optical axis in the
main-scanning plane, and the z axis extends perpendicular to the x axis
in the sub-scanning plane, the shape of the surface in a meridional
direction, which corresponds to the main-scanning direction, is expressed
as follows:

where R is the radius of curvature and K, B4, B6, B8, and
B10 are aspherical surface coefficients.

[0118]In addition, the amount of sag S' from the meridional line in the
sagittal direction that corresponds to the sub-scanning direction
(direction perpendicular to the optical axis and the main-scanning
direction) is expressed as follows:

S'32 ΣEijYi-Zj (d)

where Eij is a coefficient, and where i and j are positive integers.
In the above equation, j=2 corresponds to a spherical component in the
sub-scanning direction and j≠2 gives the amount of aspheric
deformation that defines the non-arc shape in the sub-scanning direction.

[0119]In the present exemplary embodiment, a plurality of divergent light
beams emitted from the laser source 1 are collimated by the collimating
lens 2, while the aperture diaphragm 3 limits the collimated light beams
(the amount of the light of the light beams). Then, the collimated light
beams are incident on the cylindrical lens 4 and are output without
change in the main-scanning cross section. The light beams converge in
the sub-scanning cross section, thereby forming substantially linear
images (linear images extending in the main-scanning direction) on the
deflecting surface 5a of the light deflector 5. The light beams are
reflected and deflected by the deflecting surface 5a of the light
deflector 5, pass through the first and second imaging lenses 61 and 62,
and form spot images on the photosensitive drum surface 7. The light
deflector 5 is rotated in the direction shown by the arrow A so that the
photosensitive drum surface 7 is scanned with the light beams at a
constant speed in the direction shown by the arrow B (main-scanning
direction). In this manner, an image can be recorded on the
photosensitive drum surface 7 that functions as a recording medium.

[0120]In the present exemplary embodiment, when the number of
light-emitting portions is N, the focal length of the imaging optical
system 6 in the sub-scanning direction of the collimating lens 2 is Fcol
(mm), the maximum effective image circle of the collimating lens 2 is IS
(mm), the imaging magnification of the imaging optical system 6 in the
sub-scanning direction is βFθ' and the distance between
the light beams on the a surface to be scanned 7 in the sub-scanning
direction is 25.4/DPI (mm), the following expression can be satisfied:

0.18 (mm)≦(N-1)×Fcol/(IS×βFθ×DPI)-
≦12.0 (mm) (1)

[0121]The maximum image circle IS of the collimating lens 2 refers to the
area (diameter) through which the light beams from the light-emitting
portions can be condensed and guided to the next optical element with
sufficient optical performance. In other words, when the distance between
the optical axis and the light-emitting portion farthest from the optical
axis is Ymax and the focal length of the collimating lens 2 is f, the
maximum image circle IS corresponds to a field angle ω with which
the collimating lens 2 satisfies the following equation (1a):

Ymax=ftan ω (1a)

[0122]Conditional Expression (1) shows a condition for providing good
aberration correction and/or reduction when the Vertical Cavity Surface
Emitting Laser 1 is used and the light beams are focused with a
high-definition pitch in the sub-scanning direction. When the value of
Conditional Expression (1) is below the lower limit, it can be necessary
in some circumstances to increase the arrangement area of the laser
source 1 in the main-scanning direction, which can cause jitter in the
main-scanning direction. When the value of Conditional Expression (1) is
above the upper limit, the high-definition pitch cannot be obtained and
the size of the imaging optical system 6 is increased, which leads to an
increase in the overall size of the device.

[0123]Conditional Expression (1) can also be set as follows:

0.24 (mm)≦(N-1)×Fcol/(IS×βFθ×DPI)-
≦8.78 (mm) (2)

[0124]In addition, in the present exemplary embodiment, when the focal
length of the collimating lens 2 is Fcol, the distance between the
optical axis and the light-emitting portion farthest from the optical
axis in the sub-scanning cross section is L0, the distance between
the exit surface of the second imaging lens 62 and the light deflector 5
along the optical axis direction is SI, the imaging magnification of the
incident optical system LA in the sub-scanning direction is β0,
and the F-number of the entrance side of the collimating lens 2 in the
sub-scanning cross section is Fno, the following expression can be
satisfied:

0.10<|(SI×/Fcol+β0)×LC/(SI/(Fno×β-
0×2)|<5.43 (3)

[0125]Conditional Expression (3) shows a condition for providing good
aberration correction and/or reduction. When the value of Conditional
Expression (3) is below the lower limit, the effects of the aspheric
surfaces are not sufficient and the field curvature in the sub-scanning
cross section cannot be sufficiently reduced. When the value of
Conditional Expression (3) is above the upper limit, the uniformity of
distortion (DIST) at each scan image height is degraded and the gap
between the light beams in the sub-scanning direction (sub-scan pitch)
varies depending on the scan image height.

[0126]Conditional Expression (3) can also be set as follows:

0.13<|(SI×/Fcol+β0)×LC/(SI/(Fno×β-
0×2)|<3.98 (4)

[0127]Values of Conditional Expressions (1) to (4) according to the
present exemplary embodiment are shown in Table 1-2.

[0128]In the present exemplary embodiment, a substantial portion of the
Conditional Expressions (1) to (4) are satisfied, as is clear from Table
1-2.

[0129]In Table 1-2, the pitch on the surface to be scanned (photosensitive
drum surface) in the sub-scanning direction is set to 1200 dpi, and the
number of light-emitting portions arranged in the laser source 1 is
varied from 4 to 32. The arrangement pitch in the laser source 1 is 30
μm and the arrangement direction is the same as the sub-scanning
direction (laser rotational angle is 0°).

[0130]In addition, an application example of Table 1-2 is shown in Table
1-3. In Table 1-3, the pitch on the surface to be scanned (photosensitive
drum surface) in the sub-scanning direction is set to 2400 dpi, and the
number of light-emitting portions arranged in the laser source 1 is
varied from 4 to 32. The arrangement pitch in the laser source 1 is 30
μm and the arrangement direction can be rotated around the optical
axis by 60° from the sub-scanning direction.

[0131]Also in this example, substantial portions of the Conditional
Expressions (1) to (4) are satisfied, as is clear from Table 1-3.

[0132]FIGS. 2 and 3 show aberrations obtained when the laser source 1 has
a field angle in the sub-scanning direction.

[0133]FIG. 2 illustrates a graph of the paraxial image plane in the
sub-scanning direction, where the vertical axis illustrates the paraxial
image plane in the sub-scanning direction (sub-scan image plane) and the
horizontal axis illustrates the image height (scan image height) on the
surface to be scanned in the main-scanning direction. The graph
illustrates the case in which the light-emitting portions of the laser
source 1 can be arranged such that the field angle is varied with 0.06 mm
pitch in the range of Z=0.000 mm to 0.300 mm in terms of the distance
from the optical axis of the collimating lens 2 in the sub-scanning
direction. As is clear from FIG. 2, unlike the graph shown in FIG. 21
according to the conventional structure shown, the sub-scan image plane
barely varies (the field curvature does not easily occur) even when the
sub-scan field angle of the light-emitting portion is increased.

[0134]In the conventional structure, the field curvature in the
sub-scanning direction shown in FIG. 21 is obtained as the sum of the
field curvatures in the sub-scanning direction caused by the incident
optical system (a first optical unit) including the collimating lens and
the cylindrical lens and the imaging optical system (a second optical
unit) including the imaging lens when the field angle in the sub-scanning
direction is varied.

[0135]In comparison, according to the present exemplary embodiment, a
desirable image plane can be obtained as illustrated in FIG. 2 since the
variation directions of the field curvatures caused by the incident
optical system LA and the imaging optical system 6 in the sub-scanning
direction due to the variation in the field angle in the sub-scanning
direction are opposite to each other. In other words, the field
curvatures can cancel each other.

[0136]FIG. 3 illustrates a graph of the irradiation height of the image
point on the surface to be scanned in the sub-scanning direction, where
the vertical axis illustrates the irradiation height of the image point
in the sub-scanning direction and the horizontal axis illustrates the
image height on the surface to be scanned in the main-scanning direction
(scan image height). The graph illustrates the case in which the
light-emitting portions of the laser source 1 can be arranged such that
the field angle is varied with 0.06 mm pitch in the range of Z=0.000 mm
to 0.300 mm in terms of the distance from the optical axis of the
collimating lens 2 in the sub-scanning direction.

[0137]As is clear from FIG. 3, unlike the graph shown in FIG. 22 according
to the conventional structure, the gap between the beams in the
sub-scanning direction (sub-scan pitch) barely varies depending on the
main-scan image height even when the sub-scan field angle of the
light-emitting portion is increased. Although the amount of variation in
the sub-san pitch is large in the regions where the scan image height is
large in the graph shown in FIG. 22, the pitch is uniform in the present
exemplary embodiment. In other words, the distortion (DIST) in the
sub-scanning direction is corrected or error reduced in the present
exemplary embodiment.

[0138]In the conventional structure, the distortion (DIST) in the
sub-scanning direction shown in FIG. 22 is obtained as the sum of the
distortions (DIST) in the sub-scanning direction caused by the incident
optical system (a first optical unit) including the collimating lens and
the cylindrical lens and the imaging optical system (a second optical
unit) including the imaging lens when the field angle in the sub-scanning
direction is varied.

[0139]In comparison, according to the present exemplary embodiment,
desirable scan lines with uniform image-point irradiation height can be
obtained as illustrated in FIG. 3 since the variation directions of the
distortions (DIST) caused by the incident optical system LA and the
imaging optical system 6 in the sub-scanning direction due to the
variation in the field angle in the sub-scanning direction are opposite
to each other. In other words, the distortions (DIST) cancel each other.

[0140]FIG. 4 illustrates a graph showing the positions in the sub-scanning
direction at which the principal ray of the light beam emitted from the
light-emitting portion farthest from the optical axis in the sub-scanning
cross section passes through the optical elements. The distances from the
optical axis are normalized such that the distance between the optical
axis and the light-emitting portion farthest therefrom equals 1.

[0141]As illustrated in FIG. 4, the beam is farthest from the optical axis
when the beam passes through the second imaging lens 62, and accordingly
the shape of the exit surface of the second imaging lens 62 is aspherical
in the sub-scanning cross section.

[0142]Accordingly, in the present exemplary embodiment, the aberrations
including the field curvature and the distortion (DIST) in the
sub-scanning cross section are reliably corrected and/or reduced.

[0143]In the present exemplary embodiment, the imaging optical system 6
includes two lenses. However, the present invention is not limited to
this, and the imaging optical system 6 can also be formed of a single
lens or three or more lenses. In addition, a diffractive optical element
can also be included in the imaging optical system 6. In addition, the
material of the optical elements included in the imaging optical system 6
is not limited to plastic, and glass, for example, can also be used.

Second Exemplary Embodiment

[0144]FIG. 5 illustrates a cross section of the main part of an optical
scanning apparatus according to a second exemplary embodiment of the
present invention taken along a main-scanning direction (main-scanning
cross section). In FIG. 5, components similar to those shown in FIGS. 1A
and 1B are denoted by the same reference numerals with an "a" after the
reference numeral to indicate that some of the actual optical
characteristics can be different.

[0145]The present exemplary embodiment differs from the above-described
first exemplary embodiment in that a collimating lens 20 has an
aspherical exit surface. Other structures and the optical operation
according to the present exemplary embodiment can be similar to those of
the first exemplary embodiment, and effects related to those of the first
exemplary embodiment can also be obtained in the present exemplary
embodiment.

[0146]Referring to FIG. 5, the collimating lens (condenser lens) 20, which
can have the aspherical exit surface, can function as a light-condensing
unit and converts the light beams emitted from the light source 1 into
substantially collimated light beams. In the present exemplary
embodiment, the light beams can substantially overlap each other in a
region near the diaphragm 3 irrespective of the field angle in the
sub-scanning direction and the exit surface of the collimating lens 20
that faces the diaphragm 3 has a non-arc shape in the sub-scanning cross
section. Accordingly, the effect of the aspheric surface similar to that
described above is obtained. Thus, according to the present exemplary
embodiment, the wave aberration can be reliably corrected even when the
light beams have a field angle in the sub-scanning direction.

[0147]In the present exemplary embodiment, the lens surface through which
the light beams pass at positions near each other is aspherical in the
sub-scanning cross section. Therefore, the coma aberration can be
reliably corrected and/or reduced for each of the light beams.

[0148]Table 2-1 shows data of the optical scanning apparatus according to
the present exemplary embodiment. Expressions used in Table 2-1 are
related to those used in the first exemplary embodiment. In addition,
values of Conditional Expressions (1) to (4) according to the present
exemplary embodiment are shown in Table 2-2.

[0149]The maximum image circle IS of the collimating lens 20 refers to the
area (diameter) through which the light beams from the light-emitting
portions can be condensed and guided to the next optical element with
sufficient optical performance. In other words, when the distance between
the optical axis and the light-emitting portion farthest from the optical
axis is Ymax and the focal length of the collimating lens 20 is f, the
maximum image circle IS corresponds to a field angle ω with which
the collimating lens 20 satisfies the following equation (1b):

Ymax=ftan ω (1b)

[0150]In the present exemplary embodiment, a substantial portion of the
Conditional Expressions (1) to (4) are satisfied, as is clear from Table
2-2.

[0151]In Table 2-2, the pitch on the surface to be scanned (photosensitive
drum surface) in the sub-scanning direction is set to 1200 dpi, and the
number of light-emitting portions arranged in the laser source 1 is
varied from 4 to 32. The arrangement pitch in the laser source 1 is 30
μm and the arrangement direction is the same as the sub-scanning
direction (laser rotational angle is 0°).

[0152]FIGS. 6 and 7 show aberrations obtained when the laser source 1 has
a field angle in the sub-scanning direction.

[0153]FIG. 6 illustrates a graph of the paraxial image plane in the
sub-scanning direction, where the vertical axis illustrates the paraxial
image plane in the sub-scanning direction (sub-scan image plane) and the
horizontal axis illustrates the image height (scan image height) on a
surface to be scanned in the main-scanning direction. The graph
illustrates the case in which the light-emitting portions of the laser
source 1 can be arranged such that the field angle is varied with 0.06 mm
pitch in the range of Z=0.000 mm to 0.300 mm in terms of the distance
from the optical axis of the collimating lens 20 in the sub-scanning
direction. As is clear from FIG. 6, unlike the graph shown in FIG. 21
according to the conventional structure shown, the sub-scan image plane
barely varies (the field curvature does not easily occur) even when the
sub-scan field angle of the light-emitting portion is increased.

[0154]In the conventional structure, the field curvature in the
sub-scanning direction shown in FIG. 21 is obtained as the sum of the
field curvatures in the sub-scanning direction caused by the incident
optical system (a first optical unit) including the collimating lens and
the cylindrical lens and the imaging optical system (a second optical
unit) including the imaging lens when the field angle in the sub-scanning
direction is varied.

[0155]In comparison, according to the present exemplary embodiment, a
desirable image plane can be obtained as illustrated in FIG. 6 since the
variation directions of the field curvatures caused by the incident
optical system LA and the imaging optical system 6 in the sub-scanning
direction due to the variation in the field angle in the sub-scanning
direction are opposite to each other. In other words, the field
curvatures effectively cancel or reduce each other.

[0156]FIG. 7 illustrates a graph of the irradiation height of the image
point on a surface to be scanned in the sub-scanning direction, where the
vertical axis illustrates the irradiation height of the image point in
the sub-scanning direction and the horizontal axis illustrates the image
height on a surface to be scanned in the main-scanning direction (scan
image height). The graph illustrates the case in which the light-emitting
portions of the laser source 1a can be arranged such that the field angle
is varied with 0.06 mm pitch in the range of Z=0.000 mm to 0.300 mm in
terms of the distance from the optical axis of the collimating lens 20 in
the sub-scanning direction.

[0157]As is clear from FIG. 7, unlike the graph shown in FIG. 22 according
to the conventional structure, the gap between the beams in the
sub-scanning direction (sub-scan pitch) barely varies depending on the
main-scan image height even when the sub-scan field angle of the
light-emitting portion is increased. Although the amount of variation in
the sub-san pitch is large in the regions where the scan image height is
large in the graph shown in FIG. 22, the pitch is uniform in the present
exemplary embodiment. In other words, the distortion (DIST) in the
sub-scanning direction is corrected or error reduced in the present
exemplary embodiment.

[0158]In the conventional structure, the distortion (DIST) in the
sub-scanning direction shown in FIG. 22 is obtained as the sum of the
distortions (DIST) in the sub-scanning direction caused by the incident
optical system (a first optical unit) including the collimating lens and
the cylindrical lens and the imaging optical system (a second optical
unit) including the imaging lens when the field angle in the sub-scanning
direction is varied.

[0159]In comparison, according to the present exemplary embodiment,
desirable scan lines with uniform image-point irradiation height can be
obtained as illustrated in FIG. 7 since the variation directions of the
distortions (DIST) caused by the incident optical system LA and the
imaging optical system 6a in the sub-scanning direction due to the
variation in the field angle in the sub-scanning direction are opposite
to each other. In other words, the distortions (DIST) cancel each other.

[0160]FIG. 8 illustrates a graph showing the positions in the sub-scanning
direction at which the principal ray of the light beam emitted from the
light-emitting portion farthest from the optical axis in the sub-scanning
cross section passes through the optical elements. The distances from the
optical axis are normalized such that the distance between the optical
axis and the light-emitting portion farthest therefrom equals 1.

[0161]As illustrated in FIG. 8, the beam is farthest from the optical axis
when the beam passes through the second imaging lens 62a, and accordingly
the shape of the exit surface of the second imaging lens 62a is
aspherical in the sub-scanning cross section.

Third Exemplary Embodiment

[0162]Next, an optical scanning apparatus according to a third exemplary
embodiment will be described below. The structure of the optical system
is related to that shown in FIGS. 1A and 1B and thus discussion of the
third exemplary embodiment will refer to the same reference numerals as
in FIGS. 1A and 1B but with different properties, as discussed below.

[0163]The present exemplary embodiment differs from the above-described
first exemplary embodiment in that the second imaging lens 62 formed of
the anamorphic lens has an entrance surface expressed by Expression B and
an aspherical exit surface expressed by Expression A. Other structures
and the optical operation according to the present exemplary embodiment
are related to those of the first exemplary embodiment, and effects
related to those of the first exemplary embodiment can also be obtained
in the present exemplary embodiment.

[0164]According to the present exemplary embodiment, the entrance surface
of the second imaging lens 62 is expressed by Expression B and the exit
surface of the second imaging lens 62 is aspherical and is expressed by
Expression A. The entrance surface of the second imaging lens 62 has a
non-arc shape in the sub-scanning cross section.

[0165]Table 3-1 shows data of the optical scanning apparatus according to
the present exemplary embodiment. Expressions used in Table 3-1 are
related to those used in the first exemplary embodiment.

[0166]The structure of the imaging optical system 6 and the optical
operation thereof will be described below.

[0167]The imaging optical system includes first and second imaging lenses
61 and 62 (e.g., made of resin) and causes the light beams reflected and
deflected by the light deflector 5 to form an image on the surface to be
scanned 7. Accordingly, beam spots are formed and the surface to be
scanned 7 is scanned at a constant speed.

[0168]The first and second imaging lenses 61 and 62 (e.g., made of resin)
can be manufactured by a conventional molding process in which resin is
injected into a mold and is taken out from the mold after being cooled.
Accordingly, if the imaging lenses 61 and 62 are manufactured by molding
then they can be easily manufactured with low cost compared to
conventional imaging lenses made of glass.

[0169]The first imaging lens 61 can have a positive power mainly in the
main-scanning direction as illustrated in Table 3-1, and can have an
aspherical lens surfaces with shapes expressed by Equations (a) to (d).
The first imaging lens 61 according to the present exemplary embodiment
has a higher power in the main-scanning cross section (main-scanning
direction) than in the sub-scanning cross section (sub-scanning
direction). In the main-scanning cross section, the first imaging lens 61
has a meniscus shape and the entrance surface thereof is non-arc and
concave toward the light deflector 5. In the sub-scanning cross section,
the first imaging lens 61 has a cylindrical shape in which both the
entrance and exit surfaces are flat in the sub-scanning direction.
However, it is not necessary that the entrance and exit surfaces be
completely flat, as describe in the first exemplary embodiment.

[0171]The second imaging lens 62 is an anamorphic lens having different
powers in the main-scanning direction and the sub-scanning direction, as
illustrated in Table 3-1.

[0172]The present exemplary embodiment differs from the above-described
first exemplary embodiment in that the entrance and exit surfaces of the
second imaging lens 62 are aspherical and are expressed by Expressions B
and A, respectively. The entrance surface has a non-arc shape in the
sub-scanning cross section.

[0173]The second imaging lens 62 has a higher power in the sub-scanning
cross section than in the main-scanning cross section. In the
main-scanning cross section, the entrance surface of the second imaging
lens 62 has an arc shape and the exit surface thereof has a non-arc
shape. In the sub-scanning cross section, the entrance surface of the
second imaging lens 62 has a non-arc shape and the exit surface thereof
has an arc shape.

[0174]The second imaging lens 62 can have a lens shape that is asymmetric
with respect to the optical axis in the main-scanning cross section, and
has substantially no power in the main-scanning direction in a region
around the optical axis. In the sub-scanning cross section, the entrance
surface of the second imaging lens 62 has a convex shape with a curvature
that gradually changes as the distance from the optical axis is
increased, and the exit surface has a non-arc shape that is aspherical
with respect to the optical axis.

[0175]The second imaging lens 62 focuses the light beams incident thereon
mainly in the sub-scanning direction. In addition, the second imaging
lens 62 also serves a certain distortion-correcting function in the
main-scanning direction.

[0176]The imaging optical system 6 including the first and second imaging
lenses 61 and 62 provides an imaging relationship in the sub-scanning
direction such that the imaging optical system 6 can function as a
surface-tilt correction optical system that provides a conjugate
relationship between the deflecting surface 5a of the light deflector 5
and the photosensitive drum surface 7.

[0177]It is not necessary to express the shapes of the first and second
imaging lenses 61 and 62 with the functional expressions shown in Table
3-1, and other conventional expressions can be also used.

[0178]Values of Conditional Expressions (1) to (4) according to the
present exemplary embodiment are shown in Table 3-2.

[0179]In the present exemplary embodiment, a substantial portion of the
Conditional Expressions (1) to (4) are satisfied, as is clear from Table
3-2.

[0180]In Table 3-2, the pitch on the surface to be scanned (photosensitive
drum surface) in the sub-scanning direction is set (e.g., to 1200 dpi),
and the number of light-emitting portions arranged in the laser source 1
is varied from 4 to 32. The arrangement pitch in the laser source 1 is 30
μm and the arrangement direction is the same as the sub-scanning
direction (laser rotational angle is 0°).

[0181]FIGS. 9 and 10 show aberrations obtained when the laser source 1 has
a field angle in the sub-scanning direction.

[0182]FIG. 9 illustrates a graph of the paraxial image plane in the
sub-scanning direction, where the vertical axis illustrates the paraxial
image plane in the sub-scanning direction (sub-scan image plane) and the
horizontal axis illustrates the image height (scan image height) on a
surface to be scanned in the main-scanning direction. The graph
illustrates the case in which the light-emitting portions of the laser
source 1 can be arranged such that the field angle is varied with 0.06 mm
pitch in the range of Z=0.000 mm to 0.300 mm in terms of the distance
from the optical axis of the collimating lens 2 in the sub-scanning
direction. As is clear from FIG. 9, unlike the graph shown in FIG. 21
according to the conventional structure shown, the sub-scan image plane
barely varies (the field curvature does not easily occur) even when the
sub-scan field angle of the light-emitting portion is increased.

[0183]In the conventional structure, the field curvature in the
sub-scanning direction shown in FIG. 21 is obtained as the sum of the
field curvatures in the sub-scanning direction caused by the incident
optical system (a first optical unit) including the collimating lens and
the cylindrical lens and the imaging optical system (a second optical
unit) including the imaging lens when the field angle in the sub-scanning
direction is varied.

[0184]In comparison, according to the present exemplary embodiment, a
desirable image plane can be obtained as illustrated in FIG. 9 since the
variation directions of the field curvatures caused by the incident
optical system LA and the imaging optical system 6 in the sub-scanning
direction due to the variation in the field angle in the sub-scanning
direction are opposite to each other. In other words, the field
curvatures cancel or reduce each other.

[0185]FIG. 10 illustrates a graph of the irradiation height of the image
point on the surface to be scanned in the sub-scanning direction, where
the vertical axis illustrates the irradiation height of the image point
in the sub-scanning direction and the horizontal axis illustrates the
image height on the surface to be scanned in the main-scanning direction
(scan image height). The graph illustrates the case in which the
light-emitting portions of the laser source 1 can be arranged such that
the field angle is varied with 0.06 mm pitch in the range of Z=0.000 mm
to 0.300 mm in terms of the distance from the optical axis of the
collimating lens 2 in the sub-scanning direction.

[0186]As is clear from FIG. 10, unlike the graph shown in FIG. 22
according to the conventional structure, the gap between the beams in the
sub-scanning direction (sub-scan pitch) barely varies depending on the
main-scan image height even when the sub-scan field angle of the
light-emitting portion is increased. Although the amount of variation in
the sub-san pitch is large in the regions where the scan image height is
large in the graph shown in FIG. 22, the pitch is uniform in the present
exemplary embodiment. In other words, the distortion (DIST) in the
sub-scanning direction is corrected or error reduced in the present
exemplary embodiment.

[0187]In the conventional structure, the distortion (DIST) in the
sub-scanning direction shown in FIG. 22 is obtained as the sum of the
distortions (DIST) in the sub-scanning direction caused by the incident
optical system (a first optical unit) including the collimating lens and
the cylindrical lens and the imaging optical system (a second optical
unit) including the imaging lens when the field angle in the sub-scanning
direction is varied.

[0188]In comparison, according to the present exemplary embodiment,
desirable scan lines with uniform image-point irradiation height can be
obtained as illustrated in FIG. 10 since the variation directions of the
distortions (DIST) caused by the incident optical system LA and the
imaging optical system 6 in the sub-scanning direction due to the
variation in the field angle in the sub-scanning direction are opposite
to each other. In other words, the distortions (DIST) cancel each other.

[0189]FIG. 11 illustrates a graph showing the positions in the
sub-scanning direction at which the principal ray of the light beam
emitted from the light-emitting portion farthest from the optical axis in
the sub-scanning cross section passes through the optical elements. The
distances from the optical axis are normalized such that the distance
between the optical axis and the light-emitting portion farthest
therefrom equals 1.

[0190]As illustrated in FIG. 11, the beam is farthest from the optical
axis when the beam passes through the second imaging lens 62, and
accordingly the shape of the exit surface of the second imaging lens 62
is aspherical in the sub-scanning cross section.

Fourth Exemplary Embodiment

[0191]FIG. 12 illustrates a cross section of the main part of an optical
scanning apparatus according to a fourth exemplary embodiment of the
present invention taken along a main-scanning direction (main-scanning
cross section). In FIG. 12, components related to those shown in FIGS. 1A
and 1B are denoted by the same reference numerals but with a "b" after
the numerals to signify that the components in the fourth exemplary
embodiment can have different optical values and properties than the
exemplary embodiments that refer to FIGS. 1A and 1B.

[0192]The present exemplary embodiment differs from the above-described
first exemplary embodiment in that the distance between the
light-emitting portions in the sub-scanning direction is changed. Other
structures and the optical operation of the present exemplary embodiment
are related to those of the first exemplary embodiment, and effects
related to those of the first exemplary embodiment can also be obtained
in the present exemplary embodiment.

[0193]Table 4-1 shows data of the optical scanning apparatus according to
the present exemplary embodiment. Expressions used in Table 4-1 are
related to those used in the first exemplary embodiment. In addition,
values of Conditional Expressions (1) to (4) according to the present
exemplary embodiment are shown in Table 4-2.

[0194]In the present exemplary embodiment, a substantial portion of the
Conditional Expressions (1) to (4) are satisfied, as is clear from Table
4-2.

[0195]In Table 4-2, the pitch on the surface to be scanned (photosensitive
drum surface) in the sub-scanning direction is set (e.g., to 1200 dpi),
and the number of light-emitting portions arranged in the laser source 1
is varied from 4 to 32. The arrangement pitch in the laser source 1 is 10
μm and the arrangement direction is the same as the sub-scanning
direction (laser rotational angle is 0°).

[0196]FIGS. 13 and 14 show aberrations obtained when the laser source 1
has a field angle in the sub-scanning direction.

[0197]FIG. 13 illustrates a graph of the paraxial image plane in the
sub-scanning direction, where the vertical axis illustrates the paraxial
image plane in the sub-scanning direction (sub-scan image plane) and the
horizontal axis illustrates the image height (scan image height) on a
surface to be scanned in the main-scanning direction. The graph
illustrates the case in which the light-emitting portions of the laser
source 1 can be arranged such that the field angle is varied with 0.02 mm
pitch in the range of Z=0.000 mm to 0.100 mm in terms of the distance
from the optical axis of the collimating lens 2 in the sub-scanning
direction. As is clear from FIG. 13, unlike the graph shown in FIG. 21
according to the conventional structure shown, the sub-scan image plane
barely varies (the field curvature does not easily occur) even when the
sub-scan field angle of the light-emitting portion is increased.

[0198]In the conventional structure, the field curvature in the
sub-scanning direction shown in FIG. 21 is obtained as the sum of the
field curvatures in the sub-scanning direction caused by the incident
optical system (a first optical unit) including the collimating lens and
the cylindrical lens and the imaging optical system (a second optical
unit) including the imaging lens when the field angle in the sub-scanning
direction is varied.

[0199]In comparison, according to the present exemplary embodiment, a
desirable image plane can be obtained as illustrated in FIG. 13 since the
variation directions of the field curvatures caused by the incident
optical system LA and the imaging optical system 6 in the sub-scanning
direction due to the variation in the field angle in the sub-scanning
direction are opposite to each other. In other words, the field
curvatures cancel each other.

[0200]FIG. 14 illustrates a graph of the irradiation height of the image
point on the surface to be scanned in the sub-scanning direction, where
the vertical axis illustrates the irradiation height of the image point
in the sub-scanning direction and the horizontal axis illustrates the
image height on the surface to be scanned in the main-scanning direction
(scan image height). The graph illustrates the case in which the
light-emitting portions of the laser source 1 can be arranged such that
the field angle is varied with 0.02 mm pitch in the range of Z=0.000 mm
to 0.100 mm in terms of the distance from the optical axis of the
collimating lens 2 in the sub-scanning direction.

[0201]As is clear from FIG. 14, unlike the graph shown in FIG. 22
according to the conventional structure, the gap between the beams in the
sub-scanning direction (sub-scan pitch) barely varies depending on the
main-scan image height even when the sub-scan field angle of the
light-emitting portion is increased. Although the amount of variation in
the sub-san pitch is large in the regions where the scan image height is
large in the graph shown in FIG. 22, the pitch is uniform in the present
exemplary embodiment. In other words, the distortion (DIST) in the
sub-scanning direction is corrected or error reduced in the present
exemplary embodiment.

[0202]In the conventional structure, the distortion (DIST) in the
sub-scanning direction shown in FIG. 22 is obtained as the sum of the
distortions (DIST) in the sub-scanning direction caused by the incident
optical system (a first optical unit) including the collimating lens and
the cylindrical lens and the imaging optical system (a second optical
unit) including the imaging lens when the field angle in the sub-scanning
direction is varied.

[0203]In comparison, according to the present exemplary embodiment,
desirable scan lines with uniform image-point irradiation height can be
obtained as illustrated in FIG. 14 since the variation directions of the
distortions (DIST) caused by the incident optical system LA and the
imaging optical system 6b in the sub-scanning direction due to the
variation in the field angle in the sub-scanning direction are opposite
to each other. In other words, the distortions (DIST) cancel and/or
reduce each other.

[0204]FIG. 15 illustrates a graph showing the positions in the
sub-scanning direction at which the principal ray of the light beam
emitted from the light-emitting portion farthest from the optical axis in
the sub-scanning cross section passes through the optical elements. The
distances from the optical axis are normalized such that the distance
between the optical axis and the light-emitting portion farthest
therefrom equals 1.

[0205]As illustrated in FIG. 15, the beam is farthest from the optical
axis when the beam passes through the second imaging lens 62b, and
accordingly the shape of the exit surface of the second imaging lens 62b
is aspherical in the sub-scanning cross section.

Fifth Exemplary Embodiment

[0206]Next, an optical scanning apparatus according to a fifth exemplary
embodiment will be described below. The structure of the optical system
is related to that shown in FIGS. 1A and 1B and thus discussion of the
fifth exemplary embodiment will refer to the same reference numerals as
in FIGS. 1A and 1B but with different properties, as discussed below.

[0207]The present exemplary embodiment differs from the above-described
first exemplary embodiment in that the distance between the
light-emitting portions in the sub-scanning direction is changed. Other
structures and the optical operation of the present exemplary embodiment
are related to those of the first exemplary embodiment, and effects
related to those of the first exemplary embodiment can also be obtained
in the present exemplary embodiment.

[0208]Table 5-1 shows data of the optical scanning apparatus according to
the present exemplary embodiment. Expressions used in Table 5-1 are
related to those used in the first exemplary embodiment. In addition,
values of Conditional Expressions (1) to (4) according to the present
exemplary embodiment are shown in Table 5-2.

[0209]In the present exemplary embodiment, a substantial portion of the
Conditional Expressions (1) to (4) are satisfied, as is clear from Table
5-2.

[0210]In Table 5-2, the pitch on the surface to be scanned (photosensitive
drum surface) in the sub-scanning direction is set (e.g., to 1200 dpi),
and the number of light-emitting portions arranged in the laser source 1
is varied from 4 to 32. The arrangement pitch in the laser source 1 is 10
μm and the arrangement direction is the same as the sub-scanning
direction (laser rotational angle is 0°).

[0211]FIGS. 16 and 17 show aberrations obtained when the laser source 1
has a field angle in the sub-scanning direction.

[0212]FIG. 16 illustrates a graph of the paraxial image plane in the
sub-scanning direction, where the vertical axis illustrates the paraxial
image plane in the sub-scanning direction (sub-scan image plane) and the
horizontal axis illustrates the image height (scan image height) on the a
surface to be scanned in the main-scanning direction. The graph
illustrates the case in which the light-emitting portions of the laser
source 1 can be arranged such that the field angle is varied with 0.02 mm
pitch in the range of Z=0.000 mm to 0.100 mm in terms of the distance
from the optical axis of the collimating lens 2 in the sub-scanning
direction. As is clear from FIG. 16, unlike the graph shown in FIG. 21
according to the conventional structure shown, the sub-scan image plane
barely varies (the field curvature does not easily occur) even when the
sub-scan field angle of the light-emitting portion is increased.

[0213]In the conventional structure, the field curvature in the
sub-scanning direction shown in FIG. 21 is obtained as the sum of the
field curvatures in the sub-scanning direction caused by the incident
optical system (a first optical unit) including the collimating lens and
the cylindrical lens and the imaging optical system (a second optical
unit) including the imaging lens when the field angle in the sub-scanning
direction is varied.

[0214]In comparison, according to the present exemplary embodiment, an
image plane can be obtained as illustrated in FIG. 16 since the variation
directions of the field curvatures caused by the incident optical system
LA and the imaging optical system 6 in the sub-scanning direction due to
the variation in the field angle in the sub-scanning direction are
opposite to each other. In other words, the field curvatures cancel each
other.

[0215]FIG. 17 illustrates a graph of the irradiation height of the image
point on the a surface to be scanned in the sub-scanning direction, where
the vertical axis illustrates the irradiation height of the image point
in the sub-scanning direction and the horizontal axis illustrates the
image height on the a surface to be scanned in the main-scanning
direction (scan image height). The graph illustrates the case in which
the light-emitting portions of the laser source 1 can be arranged such
that the field angle is varied with 0.02 mm pitch in the range of Z=0.000
mm to 0.100 mm in terms of the distance from the optical axis of the
collimating lens 2 in the sub-scanning direction.

[0216]As is clear from FIG. 17, unlike the graph shown in FIG. 22
according to the conventional structure, the gap between the beams in the
sub-scanning direction (sub-scan pitch) barely varies depending on the
main-scan image height even when the sub-scan field angle of the
light-emitting portion is increased. Although the amount of variation in
the sub-san pitch is large in the regions where the scan image height is
large in the graph shown in FIG. 22, the pitch is uniform in the present
exemplary embodiment. In other words, the distortion (DIST) in the
sub-scanning direction is corrected or error reduced in the present
exemplary embodiment.

[0217]In the conventional structure, the distortion (DIST) in the
sub-scanning direction shown in FIG. 22 is obtained as the sum of the
distortions (DIST) in the sub-scanning direction caused by the incident
optical system (a first optical unit) including the collimating lens and
the cylindrical lens and the imaging optical system (a second optical
unit) including the imaging lens when the field angle in the sub-scanning
direction is varied.

[0218]In comparison, according to the present exemplary embodiment,
desirable scan lines with uniform image-point irradiation height can be
obtained as illustrated in FIG. 17 since the variation directions of the
distortions (DIST) caused by the incident optical system LA and the
imaging optical system 6 in the sub-scanning direction due to the
variation in the field angle in the sub-scanning direction are opposite
to each other. In other words, the distortions (DIST) cancel and/or
reduce each other.

[0219]FIG. 18 illustrates a graph showing the positions in the
sub-scanning direction at which the principal ray of the light beam
emitted from the light-emitting portion farthest from the optical axis in
the sub-scanning cross section passes through the optical elements. The
distances from the optical axis are normalized such that the distance
between the optical axis and the light-emitting portion farthest
therefrom equals 1.

[0220]As illustrated in FIG. 18, the beam is farthest from the optical
axis when the beam passes through the second imaging lens 62, and
accordingly the shape of the exit surface of the second imaging lens 62
is aspherical in the sub-scanning cross section.

[0221]According to the first to fifth exemplary embodiments, a plurality
of light-emitting portions can be arranged one-dimensionally in the
Vertical Cavity Surface Emitting Laser, as is clear from Tables 1-2, 1-3,
2-2, 3-2, 4-2, and 5-2. However, the present invention is not limited to
this or the values provided in the illustrative examples.

[0222]The present invention can be applied to Vertical Cavity Surface
Emitting Laser in which a plurality of light-emitting portions can be
arranged two-dimensionally.

[0223]For example, a surface-emitting laser including sixteen
light-emitting portions arranged in two rows in the main-scanning
direction and eight columns in the sub-scanning direction on the same
substrate can also be used in at least one exemplary embodiment.

Image-Forming Apparatus

[0224]FIG. 19 is a cross-sectional view of the main portion of an
image-forming apparatus according to an exemplary embodiment of the
present invention taken along the sub-scanning direction. Referring to
FIG. 19, an image-forming apparatus 104 receives code data Dc from an
external device 117, (e.g., a personal computer). The code data Dc is
converted into image data (dot data) Di by a printer controller 111
included in the image-forming apparatus 104. The image data Di is input
to an optical scanning unit (optical scanning apparatus) 100, which can
have a structure according to one of the above-described first to fifth
exemplary embodiments. The optical scanning unit 100 emits a light beam
103 modulated in accordance with the image data Di and a photosensitive
surface of a photosensitive drum 101 is scanned in the main scanning
direction by the light beam 103.

[0225]The photosensitive drum 101 can function as an electrostatic latent
image carrier (photosensitive member) and is rotated (e.g., clockwise) by
a motor 115. Due to this rotation, the photosensitive surface of the
photosensitive drum 101 moves relative to the light beam 103 in the
sub-scanning direction, which is perpendicular to the main scanning
direction. A charging roller 102 for uniformly charging the surface of
the photosensitive drum 101 is provided above the photosensitive drum 101
in such a manner that the charging roller 102 is in contact with the
surface of the photosensitive drum 101. The surface of the photosensitive
drum 101 that is charged by the charging roller 102 is irradiated with
the light beam 103 emitted from the optical scanning unit 100.

[0226]As described above, the light beam 103 is modulated on the basis of
the image data Di, and the surface of the photosensitive drum 101 is
irradiated with this light beam 103 so that an electrostatic latent image
is formed thereon. The electrostatic latent image is developed as a toner
image by a developing device 107 disposed such that the developing device
107 is in contact with the photosensitive drum 101 at a position on the
downstream of the position at which the photosensitive drum 101 is
irradiated with the light beam 103 in the rotating direction of the
photosensitive drum 101.

[0227]The toner image developed by the developing device 107 is
transferred onto a paper sheet 112 that can function as a transferring
material by a transferring roller 108 disposed below the photosensitive
drum 101 so as to face the photosensitive drum 101. Although the paper
sheet 112 is fed from a paper cassette 109 disposed in front of the
photosensitive drum 101 (on the right in FIG. 19) in this example, it can
also be fed manually. A paper feed roller 110 that is disposed at an end
of the paper cassette 109 conveys the paper sheet 112 contained in the
paper cassette 109 to a transporting path.

[0228]The paper sheet 112 on which the unfixed toner image is transferred
as described above is further transported to a fixing device disposed
behind the photosensitive drum 101 (on the left in FIG. 19). The fixing
device includes a fixing roller 113, which can have a fixing heater (not
shown) therein, and a pressure roller 114 disposed so as to be in
pressure contact with the fixing roller 113. The paper sheet 112 conveyed
from the transferring section is pressed and heated in a nip portion
between the fixing roller 113 and the pressure roller 114 so that the
unfixed toner image on the paper 112 is fixed. Paper output rollers 116
are disposed behind the fixing roller 113 and the paper sheet 112 on
which the image is fixed is output from the image-forming apparatus 104.

[0229]Although not shown in FIG. 19, the printer controller 111 not only
performs the above-described data conversion but can also control
components, such as the motor 115, included in the image-forming
apparatus 104 and a light deflector, which will be described below,
included in the optical scanning unit 100.

[0230]The recording density of the image-forming apparatus according to at
least one exemplary embodiment is not particularly limited. However, the
required image quality is increased as the recording density is
increased, and therefore the structures according to the first to third
exemplary embodiments of the present invention are effective for use in
an image-forming apparatus with a recording density of 1200 dpi or more.

Color Image-Forming Apparatus

[0231]FIG. 20 is a schematic diagram illustrating the main portion of a
color image-forming apparatus according to another exemplary embodiment
of the present invention. In the present exemplary embodiment, the color
image-forming apparatus is of a tandem type in which four optical
scanning apparatus can be arranged and image information can be recorded
in parallel on surfaces of photosensitive drums that function as image
carriers. Referring to FIG. 20, a color image-forming apparatus 60
includes optical scanning apparatus 11, 12, 13 and 14, which each have
the structure according to one of the above-described first to fifth
exemplary embodiments, photosensitive drums 21, 22, 23 and 24 which each
can function as an image carrier, developing devices 31, 32, 33 and 34,
and a conveying belt 51.

[0233]In this color image-forming apparatus 60, four optical scanning
apparatuses 11, 12, 13 and 14 corresponding to cyan (C), magenta (M),
yellow (Y), and black (K), respectively, can be arranged and image
signals (image information) are recorded in parallel on the surfaces of
the photosensitive drums 21, 22, 23 and, 24, respectively. Accordingly,
color images can be printed at a high speed.

[0234]In the color image-forming apparatus 60 according to the present
exemplary embodiment, the four optical scanning apparatus 11, 12, 13, and
14 form four latent images of the respective colors on the surfaces of
the photosensitive drums 21, 22, 23 and 24 using light beams based on the
respective image data elements. Then, the images are transferred onto the
paper sheet so that a single full-color image is formed thereon.

[0235]The external device 52 can include, for example, a color image
reading apparatus, which can have a CCD sensor. In this case, a system
including the color image reading apparatus and the color image-forming
apparatus 60 can function as a color digital copying machine.

[0236]While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the discussed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all modifications, equivalent structures and functions.